A number of well-known motion illusions arise when luminance modulates next to a stationary edge (e.g., Anstis and Rogers, 1975; Gregory and Heard, 1983). Here, we reduce these phenomena to four novel elemental conditions and show how these conditions can be combined (like building blocks) to generate an infinite number of new illusory configurations.
Click on the “Elemental Conditions” button in the accompanying movie . In the top two panels, the luminance of the edge modulates next to stationary black or white center fields; in the bottom two panels , the luminance of the center modulates next to black or white stationary edges (Figure 1A shows one frame of the movie). In all four conditions, the fields appear to move even though they maintain a fixed spatial position. The apparent direction of motion may seem counter-intuitive: when the luminance of a modulating edge is similar to the luminance of the center, the motion is outward, whereas when the luminance of a modulating center is similar to the luminance of the edge, the motion is inward.

Shading is a powerful way to represent the relief of 3-dimensional objects in pictures. However, the way our vision interprets shaded images depends on the shape of their contours. Here two different contours of the same shading pattern convey two different reliefs, joined into an impossible object.

In this illusion, it appears that there is one set of black figures and one set of white figures. In fact, the two sets of figures are exactly identical. They appear different because the surrounding regions they are on cause the visual system to segment the images into layers. Thus one set appears to be white figures behind dark clouds, and the other set appears to be dark figures behind light clouds. If you cut out the figures you will see that they are identical!

Backscroll illusion is the apparent motion perceived in backgrounds of movie images that present locomotive objects such as people, animals and vehicles. In the attached movie, a human figure presents a walking gait against a counterphase grating. Although the grating has physical motion energies equally in the left and right directions, it appears to drift in a direction opposite to that of the gait.

The image is regular at the center, but the grid pattern is less regular at the peripheral parts of the images (both on the left and right edges). As you stare at the center of the grid for say 20 seconds, the regularity of the grid pattern at the center spreads into the irregular parts in the periphery. This illusion seems to indicate the preference of the visual brain to see regular patterns.

This stationary image appears to wave without effort. The elemental illusion is our revised version of the peripheral drift illusion, in which the direction of illusory motion is black-to-dark-gray and white-to-light-gray (Kitaoka and Ashida, 2003). In this image, blue and yellow correspond to dark gray and light gray, respectively.

Attention-induced brightness changes occur over bistable transparent surfaces. Fixate any of the points above and shift your attention to one disk or other without moving your eyes. The attended disk appears to change brightness. We believe that this happens because attention biases figure formation such that filling-in happens differently within the attended region than in the unattended region. In particular, the features from the overlap region spread within the boundaries of the attended figure, and not within the boundaries of the unattended region. This happens only for bistable transparent surfaces because only then is it ambiguous over which surface or layer the visual system should carry out the filling-in operation.

When sets of disks with tangential greylevel gradients are arranged in concentric circles (see image above, most observers perceive these disks moving around the centre, similar to Kitaoka’s ‘snake illusion’. This motion illusion is enhanced for large-scale and bright images and depends to a large extent to dynamic changes in the stimulus such as elicited by involuntary eye movements or blinks – fixating the centre of the pattern does abolish the illusion, whereas scanning the picture the motion sensation. A reliably effective version of this illusion, which does not require eye movements (i.e. persists when observers fixate the target in the centre of the image), can be generated by modulating the background luminance of the array of disks (see attached animated gif file ‘spin_disks.gif’). This stimulus offers the opportunity of studying this motion illusion – the percept of spinning disks in the absence of any physical displacement – in a highly controlled manner in psychophysical and physiological experiments, because it is not depending on involuntary eye movements or eye blinks. Work in preparation (Zanker 2005) will demonstrate how this illusion can be explained in terms of a two-dimensional motion detector network (2DMD, cf. Zanker & Walker, Naturwissenschaften 91, 149 – 156, 2004).